Method for fabricating a semiconductor device

ABSTRACT

A method for fabricating a semiconductor device, including forming a gate insulating film and a gate electrode film on a semiconductor substrate, and patterning the gate electrode film to form a gate electrode. A portion of the gate insulating film is removed to form an undercut region beneath the gate electrode. A buffer silicon film is formed over an entire surface of the resultant substrate to cover the gate electrode and to fill the undercut region. The buffer silicon film is selectively oxidized to form a buffer silicon oxide film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for fabricating asemiconductor device, and more particularly, to a method for fabricatinga gate electrode in a semiconductor device.

2. Description of the Related Art

In a conventional fabrication process of a semiconductor device, a gateelectrode of an MOS (Metal Oxide Semiconductor) transistor is fabricatedthrough the following steps.

A gate electrode material is formed to a predetermined thickness on theunderlying semiconductor substrate having a gate oxide film interposedtherebetween.

Next, the gate electrode material is partially etched to form the gateelectrode in a predetermined shape, i.e., a linear shape.

After that, the resultant substrate is subjected to a selectiveoxidation process, thereby repairing damage caused by the etchingprocess, assuring reliability of the gate oxide film, preventing anelectric field from being concentrated on an edge portion beneath thegate electrode, and preventing GIDL (Gate Induced Drain Leakage) due tothe gate electrode.

Generally, the gate electrode material is formed of polysilicon havingexcellent interfacing properties in a high temperature environment withrespect to the gate oxide film. However, as semiconductor devices becomemore highly integrated, the conventional polysilicon gate electrode doesnot satisfy the requirements for sheet resistance of the gate electrodeand operation speed.

Recently, a proposed method for forming a metallic gate electrode inwhich a refractory metal, for example, tungsten, is stacked on thepolysilicon gate electrode. However, since a tungsten film has anundesired active reaction with the underlying polysilicon gateelectrode, a conductive barrier film is formed between the tungsten filmand the polysilicon gate electrode.

In the method of forming a metallic gate electrode comprised ofatungsten film, a conductive film and a polysilicon gate electrode,undesirable oxidation occurs along an edge portion beneath the metallicgate electrode during the selective oxidation process and also oxygenpenetrates into an interface between the conductive barrier film and theoverlying tungsten film, thereby creating an undesired oxygen-series ofamorphous foreign particles.

Also, when a self-aligned contact technology is employed in a cellregion, a capping nitride film is formed on the tungsten film. In thiscase, oxygen penetrates into the interface between the capping nitridefilm and the underlying tungsten film during the selective oxidationprocess, thereby creating undesired foreign particles at the interfacetherebetween.

Accordingly, a disadvantage exists in the above described methods inthat undesired alien substances cause an increase in the resistance ofthe gate electrode.

Further, since oxygen is intruded into and oxidation occurs along theedge portion beneath the linear-shaped metallic gate electrode in theselective oxidation process, a relatively thick oxide film is formed inthe edge portion beneath the gate electrode, thereby causing an increasein the threshold voltage of the transistor.

Moreover, as a minimal line width of the gate electrode becomesgradually smaller, oxidation occurs over the lengths of the gateelectrode during the selective oxidation process, thereby deterioratingoperational properties of the semiconductor device.

SUMMARY OF THE INVENTION

The various exemplary embodiments of the present invention are directedto a method for fabricating a semiconductor device that substantiallyobviates one or more problems due to limitations and disadvantages ofthe conventional art.

A method is provided for fabricating a semiconductor device in which areliable selective oxidation process can be performed.

A method for fabricating a semiconductor device according to the presentinvention is characterized in that an oxidation stop layer (or oxygendiffusion-preventive layer) is formed prior to a selective oxidationprocess or during the selective oxidation process.

A method of fabricating a semiconductor device according to anotherembodiment of the invention includes removing a portion of a gateinsulating film to form an undercut region beneath a gate electrode,that is, at an edge portion beneath the gate electrode. A buffer siliconfilm is formed as an oxidation stop layer on the resultant substrate tocover the gate electrode and the undercut region. After the buffersilicon film is formed, a selective oxidation process is performed tocure a defect generated at the time of the etching process for formingthe gate electrode.

In a fabricating method according to another embodiment of theinvention, the buffer silicon film prevents the oxygen from penetratinginto the gate electrode in the selective oxidation process. This isbecause the buffer silicon film itself is combined with the oxygen to beoxidized in the selective oxidation process.

Further, the undercut region forms a physical penetration distancetoward a portion beneath the gate electrode that is increased as long asthe undercut region, so that oxidation can be more effectively preventedbeneath the gate electrode (e.g., at the center of the channel region).

In the fabricating method according to another embodiment of theinvention, it is desirable that the portion of the gate insulating filmis removed using a wet etching process. The etching solution can be anetching solution generally used in the etching process of the oxidationfilm.

The gate electrode is formed by depositing a polysilicon film, arefractory metallic film and a capping nitride film on the gateinsulating film and patterning the deposited films. The refractory metalfilm is formed so as to decrease the resistance of the gate electrode,which in turn improves an operation speed of the device. For example,the refractory metal film includes tungsten, titanium, tantalum,molybdenum, cobalt, magnesium, nickel and copper.

It is desirable that a barrier metallic film is further formed betweenthe polysilicon film and the tungsten film. The barrier metallic filmprevents a reaction between the polysilicon film and the tungsten film.For example, the barrier metallic film can be formed of a tungstennitride film, a titanium nitride film, etc.

Further, in the selective oxidation process, a tungsten silicide filmcan be further formed between a surface of the tungsten film exposed ina gate sidewall portion and the buffer silicon film contactingtherewith. In this case, a resistance characteristic of the gateelectrode is further improved.

Prior to the selective oxidation process, the etch-back process may befurther performed for the buffer silicon film. Accordingly, a buffersilicon spacer is formed on both sidewalls of the gate electrode. Inthis case, the buffer silicon spacer is oxidized through the selectiveoxidation process and thus oxygen is prevented from penetrating throughthe sidewall of the gate electrode.

A method of fabricating a semiconductor device according to anotherembodiment of the invention further includes performing a lightly-dopedimplantation to form a lightly-doped impurity region. This is to form alightly-doped drain region. The conductive type of the implantedimpurity ion is opposite to that of the semiconductor substrate. Forexample, if the semiconductor substrate is P-type conductive, theimplanted impurity ion is N-type conductive. The gate electrode and theoxidized buffer silicon film formed on the sidewall thereof are used asan implant mask. Accordingly, the lightly-doped impurity region ispositioned at both sides of the gate electrode, and in more detail, inthe semiconductor substrate at sides of the oxidized buffer silicon filmformed on both sidewalls of the gate electrode.

A fabricating method of the semiconductor device according to anotherembodiment of the invention further includes forming and etching-back aspacer nitride film to form a nitride spacer on both sidewalls of thegate electrode, and performing a heavily-doped implanting process toform a heavily-doped impurity region. Resultantly, the oxidized buffersilicon film and the nitride spacer are positioned on the sidewalls ofthe gate electrode to form a dual spacer.

At the time of the etch-back process for the nitride spacer, since theoxidized buffer silicon film is formed on the semiconductor substrate, aprocess margin can be increased. The heavily-doped implantation processuses the same impurity type as that of the lightly-doped implantingprocess, with a relatively higher concentration. The heavily-dopedimpurity region is formed in the semiconductor substrate, next to thelightly-doped impurity region. As a result, the lightly-doped impurityregion is defined in the semiconductor substrate and beneath the nitridespacer. According to exemplary embodiments of the invention, the buffersilicon film used as an oxidation stop layer in the selective oxidationprocess simultaneously forms the dual spacer. That is, the oxidationstop layer formed for the selective oxidation process is used as aninsulating film required when the dual spacer is formed. Accordingly, itis not required to form an additional film for forming the dual spacer.As a result, a simplified and economic fabricating process can beperformed.

Further, the lightly-doped impurity region is spaced apart by athickness of the oxidized buffer silicon from the sidewall of the gateelectrode. Thus, the lightly-doped impurity regions are positionedspaced apart from each other with a distance as long as a sum of alength of the gate electrode and two times the thickness of the buffersilicon film. In other words, a length of a channel region is formedlarger than a length of the gate electrode. Accordingly, disadvantagesdue to a short channel effect can be minimized.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects of the present invention will become moreapparent by describing in detail preferred embodiments thereof withreference to the attached drawings in which:

FIGS. 1 to 7 are cross-sectional views illustrating a method forfabricating a gate electrode in a semiconductor device according to anembodiment of the present invention; and

FIGS. 8 to 10 are cross-sectional views illustrating a method forfabricating a gate electrode in a semiconductor device according toanother embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. However, the present invention is not limited to theembodiments illustrated hereinafter, and the embodiments herein arerather introduced to provide easy and complete understanding of thescope and spirit of the present invention.

The various exemplary embodiments of the method of fabricating asemiconductor device according to the invention minimizes oxidation of aconductive film in the fabricated semiconductor device. Though a gateelectrode and a word line is described in the preferred embodiments, itwill be obvious to those skilled in the art that the inventive methodcan be effectively applied to stopping oxidation of other conductivestructures that are easily oxidized.

FIGS. 1 to 7 are cross-sectional views illustrating a fabricating methodof a semiconductor device according to a first embodiment of the presentinvention. For drawing simplification and description convenience, onlyone gate electrode is illustrated in the drawings. In the drawings,thickness of layers and regions are exaggerated for clarity. Further, incase it is referred that an arbitrary layer (or film) is formed “on” or“over” another layer or substrate, it implies that the arbitrary layercan be formed in contact with another layer or substrate or a thirdlayer can be interposed therebetween. Identical reference numeralsrepresent identical structural elements.

Referring to FIG. 1, after an active region is defined by a deviceisolation process, a gate insulating film 120, a polysilicon film 140, abarrier metal film 160, a refractory metal film 180 and a cappingnitride film 200 are sequentially formed on a semiconductor substrate100. The gate insulating film 120 is formed of silicon oxide forinsulation between a gate electrode and the underlying semiconductorsubstrate 100 through a thermal oxidation process. The polysilicon film140 has an excellent interfacial property in a high temperatureatmosphere with respect to the gate insulating film 120. The refractorymetal film 180 is formed to decrease the resistance of the gateelectrode. For example, the refractory metal film 180 is formed of atleast one material selected from a group consisting of tungsten,titanium, tantalum, molybdenum, cobalt, magnesium, nickel and copper.Preferably, the refractory metal film 180 is formed of tungsten.

The barrier metal film 160 is optionally formed so as to prevent areaction between the polysilicon film 140 and the overlying tungstenfilm 180. For example, the barrier metal film 160 can be formed oftungsten nitride, titanium nitride or the like.

Referring to FIG. 2, the capping nitride film 200, the tungsten film180, the barrier metal film 160 and the polysilicon film 140 aresequentially patterned to form the gate electrode 220, an upper portionof which is protected by the patterned capping nitride film 200 a. As aresult, the gate electrode 220 includes the polysilicon film 140 a, thebarrier metal film 160 a and the tungsten film 180 a. The patterningprocess is performed through the steps of forming a photo-sensitivephotoresist film on the capping nitride film 200, patterning thephotoresist film through a light-exposure process and a developingprocess to form a photoresist pattern, and etching the underlyingexposed films using the photoresist pattern as an etching mask. At thistime, etching of the underlying films 140, 160, 180 and 200 is performeduntil the gate insulating film 120 is exposed.

Referring to FIG. 3, the exposed gate insulating film 120 is partlyremoved from the resultant substrate, so that an undercut region 230 isformed at an edge portion beneath the gate electrode 220. The gateinsulating film 120 on the semiconductor substrate 100 and the gateinsulating film 120 beneath the gate electrode 220 are concurrentlywet-etched and removed from the resultant substrate, thereby forming aremnant gate insulating film 120 a beneath the gate electrode 220. As aresult, predetermined portions of the resultant substrate facing thebottom edges of the gate electrode 220 are exposed and at the same timeboth bottom edges of the gate electrode 220 are also exposed.

Referring to FIG. 4, a buffer silicon film 240 is formed on an entiresurface of the resultant substrate so as to cover the gate electrode 220including the exposed undercut region 230. In other words, the buffersilicon film 240 is formed on the gate electrode 220 and the exposedsemiconductor substrate 100 with a regular thickness while filling theexposed undercut region 230. Accordingly, the sidewalls of the gateelectrode 220 are protected. The buffer silicon film 240 can be formedbelow a thickness of about 200 Å.

Referring to FIG. 5, a selective oxidation process is performed torecover damage due to the etching process of the gate electrode 220 andto improve the operation property of the semiconductor device. Since thebuffer silicon film 240 is formed on the sidewalls of the gate electrode220 and in the undercut region 230, oxygen is prevented from penetratinginto the gate electrode 220 during the selective oxidation process. Thatis, in the selective oxidation process, the buffer silicon film 240reacts with oxygen (that is, oxygen consumption) and is oxidized andtransformed into a buffer silicon oxide film, i.e., a silicon oxide film260, thereby functioning as an oxidation stop layer.

During the selective oxidation process, the tungsten film 180 a exposedon the sidewalls of the gate electrode 220 reacts with the buffersilicon film 240 in contact with the tungsten film 180 a to further forma tungsten silicide film. The tungsten silicide film causes theresistance of the gate electrode to be decreased.

Further, the buffer silicon oxide film 260 is also used as an insulatingfilm for a dual spacer forming process.

Referring again to FIG. 5, after the selective oxidation process, alightly doped implantation 270 is performed to form a lightly-dopedimpurity region (that is, Lightly Doped Drain: LDD) region 280 in thesemiconductor substrate 100 at both sides of the gate electrode 220. Atthis time, an impurity ion is implanted that is a conductive typeopposite to that of the semiconductor substrate 100. For example, incase the semiconductor substrate 100 is a P-type, the implanted impurityion is an N-type. In case the semiconductor substrate 100 is an N-type,the impurity ion implanted is a P-type. The lightly-doped impurityregion 280 is positioned spaced as much as the thickness of buffersilicon oxide film 260 from both sides of the gate electrode 220. Thispositional configuration of the lightly-doped impurity region 280 is dueto the impurity ion passing through the buffer silicon oxide film 260formed on the underlying semiconductor substrate 100 and implanting intothe semiconductor substrate 100, and the gate electrode 220 and thebuffer silicon oxide film 260 formed on the sidewalls thereoffunctioning as the implant masks.

A problem due to a short channel effect can be eliminated because thelightly-doped impurity region 280 is positioned so as to be spaced by apredetermined distance from both sides of the gate electrode 220.

Referring to FIG. 6, a spacer nitride film 300 is formed on the buffersilicon oxide film 260. The spacer nitride film 300 is formed of siliconnitride by Chemical Vapor Deposition (CVD).

Referring to FIG. 7, the spacer nitride film 300 is etched-back to forma nitride spacer 300 a remaining only at both sides of the gateelectrode 220. For example, the nitride spacer 300 a is formed on thebuffer silicon oxide film 260 formed on the sidewalls of the gateelectrode 220. Thus, the gate electrode 220 having a dual spacercomprised of the nitride spacer 300 a and the buffer silicon oxide film260 is formed. The buffer silicon oxide film 260 formed on thesemiconductor substrate 100 at both sides of the gate electrode 220increases a process margin for the etch-back process.

A heavily-doped implanting process 310 for the formation of asource/drain region is performed to form a heavily-doped impurity region320. At this time, the gate electrode 220, the buffer silicon oxide film260 formed on the sidewalls of the gate electrode 220, and the nitridespacer 300 a are used as the implant masks in the heavily-dopedimplanting process 310. As a result, the heavily-doped impurity region320 is formed in the semiconductor substrate 100 at both sides of outersidewalls of the nitride spacer 300 a, so that the lightly-dopedimpurity region 280 is defined in the semiconductor substrate 100beneath the nitride spacer 300 a. The heavily-doped impurity region 320is formed in succession to the lightly-doped impurity region 280 so asto have a deeper distribution and a higher concentration of impuritythan the lightly-doped impurity region 280.

As a result, the source/drain region 320 (e.g., heavily-doped impurityregion) is completed including the LDD region 280 (e.g., lightly-dopedimpurity region).

As described above, in the fabrication methods of the semiconductordevice according to exemplary embodiments of the present invention,since the buffer silicon film 240 is oxidized to function as theoxidation stop layer during the selective oxidation process, undesiredoxidation of the gate electrode does not occur. Additionally, since theundercut region 230 is formed beneath the gate electrode 220, theoxidation stop effect can be maximized.

Further, since the film serving as the oxidation stop layer during theselective oxidation process is also used as the dual spacer,simplification and economical efficiency of the process can be improved.

FIGS. 8 to 10 show an alternative embodiment. The process of thisembodiment is similar to the process described with reference to FIGS. 1to 4. After the gate electrode 220 is formed on the semiconductorsubstrate 100, the undercut region 230 and the buffer silicon film 240are formed. (See FIG. 4).

Next, referring to FIG. 8, the buffer silicon film 240 is etched-back sothat a buffer silicon spacer 240 a covering both sidewalls of the gateelectrode 220 and filling the undercut region 230 is formed.

Referring to FIG. 9, a selective oxidation process is performed torecover damage due to the etching process of the gate electrode 220 andto improve operation properties of the semiconductor device. At thistime, since the buffer silicon spacer 240 a is formed on the undercutregion 230, oxygen is prevented from penetrating into the gate electrode220 during the selective oxidation process. During the selectiveoxidation process, the buffer silicon spacer 240 a reacts with oxygen(that is, oxygen consumption) and is oxidized and transformed into asilicon oxide spacer 260 a, thereby functioning as the oxidation stoplayer. After that, using the same process as in the embodiment describedfor FIGS. 1 to 4, the lightly-doped impurity region 280 is formed.

Next, referring to FIG. 10, using the same process as in the embodimentdescribed for FIGS. 1 to 4, the nitride spacer 300 a and thehighly-doped impurity region 320 are sequentially formed.

As described above, in the fabrication methods of the semiconductordevice according to exemplary embodiments of the present invention,since the buffer silicon film is oxidized to function as the oxidationstop layer during the selective oxidation process, undesired oxidationof the gate electrode does not occur.

Further, since the undercut region is formed beneath the gate electrode,the oxidation stop effect can be maximized.

Furthermore, since the film functioning as the oxidation stop layerduring the selective oxidation process is also used as the dual spacer,simplification and economical efficiency of the process is improved.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present invention. Thus,it is intended that the present invention covers the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1. A method for fabricating a semiconductor device, the methodcomprising the steps of: forming a gate insulating film and a gateelectrode film on a semiconductor substrate; patterning the gateelectrode film to form a gate electrode; removing a portion of the gateinsulating film to form an undercut region beneath the gate electrode;forming a buffer silicon film on an entire surface of the resultantsubstrate, the buffer silicon film covering the gate electrode andfilling the undercut region; and selectively oxidizing the buffersilicon film to form a buffer silicon oxide film.
 2. The method of claim1, wherein the portion of the gate insulating film is removed using awet etch process.
 3. The method of claim 1, wherein the buffer siliconfilm is oxidized to function as an oxidation stop layer for preventingthe gate electrode from being oxidized.
 4. The method of claim 1,wherein the gate electrode film forming step comprises the steps of:forming a silicon film on the gate insulating film; forming a refractorymetal film on the silicon film; and forming a capping nitride film onthe refractory metal film.
 5. The method of claim 4, wherein therefractory metal film is formed of at least one selected from a groupconsisting of tungsten, titanium, tantalum, molybdenum, cobalt,magnesium, nickel and copper.
 6. The method of claim 4, furthercomprising the step of forming a barrier metal film on the silicon filmprior to forming the refractory metal film.
 7. The method of claim 1,further comprising the steps of: performing a lightly-doped implantationto form a lightly-doped impurity region in the semiconductor substrateat both sides of the buffer silicon oxide film formed on sidewalls ofthe gate electrode; forming a spacer nitride film on the buffer siliconoxide film; etching-back the spacer nitride film to form a nitridespacer only on the sidewalls of the buffer silicon oxide film formed onthe sidewalls of the gate electrode; and performing a heavily-dopedimplantation into the resultant substrate to form a heavily-dopedimpurity region next to the lightly-doped impurity region, in thesemiconductor substrate outside the nitride spacer.
 8. The method ofclaim 7, further comprising the step of etching-back the buffer siliconfilm so as to allow the buffer silicon film to remain only on thesidewalls of the gate electrode and in the undercut region.
 9. A methodfor fabricating a semiconductor device, comprising the steps of: forminga gate insulating film and a gate electrode on a semiconductorsubstrate; etching and removing the gate insulating film exposed at bothsides of the gate electrode and at an edge portion of the gateinsulating film beneath the gate electrode, to thereby form an undercutregion beneath the gate electrode; forming a buffer silicon film on anentire surface of the resultant substrate; and selectively oxidizing thebuffer silicon film to form a buffer silicon oxide film.
 10. The methodof claim 9, further comprising the steps of: performing a lightly dopedimplantation into the buffer silicon oxide film to form a lightly-dopedimpurity region in the semiconductor substrate at both sides of thebuffer silicon oxide film formed on sidewalls of the gate electrode;forming a spacer nitride film on the buffer silicon oxide film;etching-back the spacer nitride film to form a nitride spacer only onthe sidewalls of the buffer silicon oxide film formed on the sidewallsof the gate electrode; and performing a heavily-doped implantation intothe resultant substrate to form a heavily-doped impurity region next tothe lightly-doped impurity region, in the semiconductor substrateoutside the nitride spacer.
 11. The method of claim 9, wherein the gateelectrode forming step comprises the steps of: forming a silicon film, atungsten film and a capping nitride film on the gate insulating film;and patterning the capping nitride film, the tungsten film and thesilicon film.
 12. The method of claim 11, further comprising the step offorming a barrier metal film between the silicon film and the tungstenfilm.
 13. The method of claim 9, wherein the buffer silicon film isoxidized to act as an oxidation stop layer, thereby preventing the gateelectrode from being oxidized.
 14. The method of claim 9, wherein theportion of the gate insulating film is removed using a wet etch process.15. A method for fabricating a semiconductor device, comprising thesteps of: forming a gate insulating film, a silicon film, a barriermetal film, a tungsten film and a capping nitride film on asemiconductor substrate; patterning the capping nitride film, thetungsten film, the barrier metal film and the silicon film to form agate electrode; wet-etching and removing the gate insulating filmexposed at both sides of the gate electrode, and at an edge portion ofthe gate insulating film beneath the gate electrode, to form an undercutregion beneath the gate electrode; forming a buffer silicon film on anentire surface of the resultant substrate in which the undercut regionis formed; selectively oxidizing the buffer silicon film to form abuffer silicon oxide film; performing a lightly-doped implantation intothe buffer silicon oxide film to form a lightly-doped impurity region inthe semiconductor substrate at both sides of the buffer silicon oxidefilm formed on sidewalls of the gate electrode; forming a spacer nitridefilm on the buffer silicon oxide film; etching-back the spacer nitridefilm to form a nitride spacer; and performing a heavily-doped implantinginto the resultant substrate to form a heavily-doped impurity regionnext to the lightly-doped impurity region, in the semiconductorsubstrate outside the nitride spacer.
 16. The method of claim 15,further comprising the step of etching-back the buffer silicon film soas to allow the buffer silicon film to remain only on the sidewalls ofthe gate electrode and in the undercut region.
 17. The method of claim15, wherein the buffer silicon film is oxidized to act as an oxidationstop layer, thereby preventing the gate electrode from being oxidized.18. A semiconductor device, comprising: a semiconductor substrate; agate electrode formed over the substrate, the gate electrode including agate insulating film, a portion of the gate insulating film beingremoved to form an undercut region beneath the gate electrode; and abuffer silicon oxide film formed over sidewalls of the gate electrodeand within the undercut region.
 19. The semiconductor device of claim18, further comprising: lightly-doped impurity regions formed in thesemiconductor substrate at sides of the buffer silicon oxide film. 20.The semiconductor device of claim 18, further comprising; nitridespacers formed on sidewalls of the buffer silicon oxide film; andheavily doped impurity regions formed next to the lightly-doped impurityregions in the semiconductor substrate at sides of the nitride spacers.